Grain-Boundary Sliding and Grain Interlocking in the Creep Deformation of Two-Phase Ceramics

Grain-boundary sliding and grain interlocking of two-phase ceramics during creep are examined on the basis of the Dryden–Kucerovsky–Wilkinson–Watt theory. That theory is extended to the plane–strain creep deformation of model arrays of square and hexagonal grains embedded in a continuous grain-boundary melt phase to develop consti-tutive equations for their universal creep. Superposition is derived for the "time-applied stress" and the "time–temperature" relations during creep. The "shift factor" for the time–temperature superposition characterizes the temperature dependence of the apparent viscosity of the ceramic. A two-phase ceramic during creep acts as a non-Newtonian fluid, the viscosity of which is dependent on the creep strain. The constitutive relation between the creep strain rate and the creep strain is then utilized to estimate the viscosity and the volume fraction of the intergranular melt phase. The equilibrium creep strain to achieve grain interlocking is considered through the rotational motion of grains. Creep tests for a β-spodumene glass-ceramic are conducted under simple shear to generate experimental results for scrutinizing the theoretical predictions. Satisfactory agreement is observed, giving important rheological information on the creep process.